Throughout the specification, including in the claims, the phrase "spatial form of a field" (and variations thereon) is used to denote parameters of a field other than a scaling factor for its amplitude (or the amplitude of one or more periodic components thereof) and the phase of one or more periodic components thereof. For example, consider a quadrupole trapping field resulting from application of an RF sinusoidal voltage (having peak-to-peak amplitude V, frequency .omega., and a phase) and optionally also a DC voltage, between the ring electrode and one of the end electrodes of a conventional three-dimensional quadrupole ion trap. Two such quadrupole trapping fields (both applied between the ring electrode and an end electrode) will have the same "spatial form" despite differences in their frequencies, phases, DC amplitudes, and/or the peak-to-peak amplitudes of their sinusoidal or other periodic components. However, a supplemental field resulting from application of a sinusoidal or other periodic voltage (and optionally also a DC component) across the end electrodes of a quadrupole trap will have a different spatial form than a quadrupole trapping field (applied between the ring electrode and an end electrode of the trap) due to the different geometries of the ring electrode and the end electrodes.
Throughout the specification, including in the claims, the expression "changing a field," and variations thereon, are used in a broad sense to denote any operation in which at least one parameter of the field is changed, including for example, performing a continuous sweep or scan of at least one parameter of the field, performing a discontinuous or pulsed application of a component of the field, or performing a discontinuous or pulsed variation of at least one parameter of the field.
Each of the expressions "trapping field" and "supplemental field" employed herein denotes a field having at least one periodically varying component. Each periodically varying component can be, but need not be, a sinusoidally varying component.
In some conventional mass spectrometry techniques, a combined field (comprising a trapping field and a supplemental field having different spatial form than the trapping field) is established in an ion trap, and the combined field is changed to excite trapped ions for detection. For example, U.S. Pat. No. 3,065,640 (issued Nov. 27, 1962) describes a three-dimensional quadrupole ion trap (with reference to FIG. 1). It teaches application of DC voltage .sup.2 V.sub.dc and AC voltage .sup.2 V.sub.ac across the trap's end electrode 13 and ring electrode 11 to establish a quadrupole trapping field in the trap, application of a supplemental voltage (having DC component V.sub.g and AC component 2V.sub..beta.) across the quadrupole trap's end electrodes 12 and 13 to establish a supplemental field in the trap (having different spatial form than the simultaneously applied quadrupole trapping field), and changing of the combined fields by increasing one or both of simultaneously applied voltages V.sub.g and V.sub.dc to eject trapped ions from the trap through a hole 25 through end electrode 12 for detection at an external detector 26 (see col. 3, lines 13-18, and col. 9, lines 9-23).
U.S. Pat. No. 3,065,640 also describes simultaneous establishment of two fields having identical spatial form in the ion trap (the quadrupole trapping field established by "drive" oscillator 18 and DC voltage source 19, and the field established by "pump" oscillator 20 which is connected in series with oscillator 18 and source 19). However, this reference does not suggest changing parameters of two superimposed fields of identical spatial form to excite trapped ions sequentially for detection.
Similarly, U.S. Pat. No. 2,939,952, issued Jun. 7, 1960, suggests (at column 6, lines 17-33) simultaneous establishment of two fields having the same spatial form in an ion trap, but does not disclose or suggest changing parameters of two fields having the same spatial form for the purpose of exciting trapped ions sequentially for detection.
In a class of conventional mass spectrometry techniques known as "MS/MS" methods, ions (known as "parent ions") having mass-to-charge ratio (hereinafter denoted as "m/z") within a selected range are isolated in an ion trap. The trapped parent ions are then allowed or induced to dissociate (for example, by colliding with background gas molecules within the trap) to produce ions known as "daughter ions." The daughter ions are then ejected from the trap and detected.
For example, U.S. Pat. No. 4,736,101, issued Apr. 5, 1988, to Syka, et al., discloses an MS/MS method in which ions (having m/z's within a predetermined range) are trapped within a three-dimensional quadrupole trapping field (established by applying a trapping voltage across the ring and end electrodes of a quadrupole ion trap). The trapping field is then scanned to eject unwanted parent ions (ions other than parent ions having a desired m/z) consecutively from the trap. The trapping field is then changed again to become capable of storing daughter ions of interest. The trapped parent ions are then induced to dissociate to produce daughter ions, and the daughter ions are ejected consecutively (sequentially by mass-to-charge ratio) from the trap for detection. U.S. Pat. No. 4,736,101 teaches (at column 5, lines 16-42) establishment of a supplemental AC field (having different spatial form than the trapping field) in the trap after the dissociation period, while the trapping voltage is scanned (or while the trapping voltage is held fixed and the frequency of the supplemental AC field is scanned). The frequency of the supplemental AC field is chosen to equal one of the components of the frequency spectrum of ion oscillation, and the supplemental AC field (if it has sufficient amplitude) thus resonantly and sequentially ejects stably trapped ions from the trap as the frequency of each ion (in the changing combined field) matches the frequency of the supplemental AC field.
U.S. Pat. No. 4,761,545, issued Aug. 2, 1988 to Marshall, et al., describes application of a variety of tailored excitation voltage signals to ion traps, including ion cyclotron resonance and quadrupole traps. The tailored excitation voltages have multiple frequency components, and can (through a three step, or optionally five step, tailored computational procedure) have any of a variety of waveforms.